Find my iPhone includes a feature called 'Activation Lock' which is designed to prevent anyone else using the device if it's ever lost or stolen. This feature stops anyone else using the device whether it's an iPhone, iPad or iWatch by requiring the owner's Apple ID and password prior to activating the device, even if the device is reset. Activation Lock is enabled automatically when you turn find my iphone on a device using ios 7 or later.

Unfortunately this means an innocent consumer may have acquired an iPhone, iPad or iWatch and are stuck with an unusable device with no way of contacting the original owner.
Unlockster can fix this for you